The present invention relates to characterization of material physical properties, and in particular, methods to dynamically induce shear in material samples at high strain rates and under conditions similar to those during very high velocity impacts, dynamic interactions, and penetration into targets. This invention further relates to techniques to perform metallurgical observation of sheared regions within the samples, testing of samples and subsamples containing the sheared regions, analytical modeling of the dynamic shearing process and development of associated constitutive relations.
Conventional dynamic shear testing apparatus used to generate high strain rate property data primarily include Kolsky Bar Impact, Split-Hopkinson Bar Impact and Pressure-Shear Plate Impact techniques. In addition, however, various non-standard techniques have been used for specific purposes and some of these have been summarized in the literature by Meyers and Murr. With several of these techniques, the material sample can be subjected to high strain rates associated with impacts only up to the lower end of ordnance velocity ( less than 1000 m/s). In a number of such tests, the temperature can be set in advance by heating the sample prior to impact. This method artificially creates a temperature that has little relation to the adiabatically induced temperatures that are generated at strain rates and compression levels during very high velocity impact and penetration. Although somewhat high pressures can be arranged for within the Hopkinson Bar and Pressure-Plate Impact techniques, there are severe limitations. For example, generally, the magnitudes and time duration of the pressure pulses resulting from the impact are functions of the apparatus and thus are severely limited by impact velocity, compression wave speeds within the sample material and sample dimensions. Further, the pressure levels, pulse lengths, and strain rates are generally far below those associated with very high velocity impact and penetration. Not only are the conditions at higher velocity impacts ( greater than 1500 m/s) not attainable, but also, the sample material is not simultaneously subjected to the high levels of dynamic pressure, temperature and rates of strain that commonly and naturally arise during ballistic impact and penetration.
Various laboratory techniques have been developed for observing and characterizing the shear zones within sheared samples. These techniques have included mechanical property tests of the bulk samples containing shear zones and metallography. With these techniques, often multiple sets of individual shears and/or regions of combined shears are examined together, such that only bulk properties rather than those of the individual shear are determined. Further, the prior art techniques most likely create shears within the sample whose directions are often at random, unknown, or in non-preferred directions.
In the past, the modeling of sheared material necessarily attempted to describe mechanical properties of the bulk material. Thus, such models as Johnson and Cook, for example, are macroscopic and require extensive measurements of many coefficients to cover the many variables associated with the number of sheared areas, multiplicity of shears within an area, shear-shear interactions, variations with impact conditions, average orientation of the shear, and distributions about the averages. Generally, these details are not known within the macroscopic framework of the model.
The existing constitutive models are based on properties of the bulk material in the macroscopic sense. While such models as proposed in Johnson and Cook include material yielding criteria, strain-rate effects, temperature influences, thermal softening, and failure criteria, the associated coefficients are numerous and are empirically based. For a fully developed constitutive relation using the prior art, numerous, extensive and expensive tests, under impact conditions to include wide ranges of the above variables, must be conducted. Further, even with these complexities, resulting relations only approximate, often not very well, the actual response of the material subject to impact and penetration since loading conditions are most often radically different from those used to make the measurements.
The present invention provides for localized singular shear structures and shear regions to be created within the test sample during very high velocity impact and penetration. Thus, the associated strain rates, strains, pressures, and temperatures are concomitant and inherent as the shear takes place during the dynamic impact process. Further, the geometry is such that the shear direction with respect to a characteristic direction within the test sample is known and can be controlled during the impact. This present technique enables direct and unambiguous knowledge of the stress state within the sample during dynamic interaction with the receptor or target and the stress state""s unambiguous relationship to the resulting shear. The details of observation conducted on recovered test samples allow for a microscopic characterization of single or multiple shears within the sample and, through subsection, allow for determination of post mechanical properties of the sheared material with respect to any desired shear orientation within the recovered sample material. The model addresses microscopic detail with generalization to a macroscopic description such that a more complete and applicable constitutive relation can be constructed.
To facilitate the shear characterization, modeling, and constitutive relation development, the geometries of the samples and impacted materials of the present invention are simulated with hydrocode computations using codes such as 2-D and 3-D CTH or Autodyn. Thus, the present invention provides for simulation of the dynamic impact using material elements in grid, cell or point mass form suitable for numerical solution. These calculations are conducted within the context of the invention to define impact experiments and associated material geometries. Further, application of the numerical computational process is used to define the stress states, strains, strain rate, pressures, temperatures, and the flow of the material during impact. The hydrocode computational process is applied iteratively at each stage of the dynamic shear characterization to analyze individual shear formation, behavior of shear zones, modeling of the phenomena, and development of constitutive relations.
It is therefore a primary object of this invention to provide a dynamic impact technique that can subject sample materials to shear along given directions within the sample and produce the shear under the conditions of strain rate, pressure, and temperature that exist during very high velocity impact- and penetration.
A second objective is to provide for post recovery examination of shear oriented samples and subsamples to determine basic material shear response to the loading conditions imposed on the sample under the conditions of impact and dynamic interaction with the receptor.
A third objective is to provide for a micromechanical model of the shear mechanism, instability, and material failure to facilitate accuracy of the description and lessen its cost.
A fourth object is to provide for geometric descriptions of the materials involved in the impact to include samples and receptors such that hydrocodes in 2-D and 3-D can be used to define tests, examine the loading conditions, calibrate the analytic models, and on an iterative basis, characterize the dynamic shearing to process, and develop constitutive relations for materials under dynamic loading conditions.
A still further object is to provide for an accurate and more applicable macroscopic constitutive relation to be used in ultimate descriptions of material response under dynamic loading. The techniques of the present invention lead to improved ability to conduct dynamic calculations, examine structural responses, assess crash worthiness, describe impact phenomena and ascertain important material properties for ballistic impact and penetration.
These and other objectives are achieved by designing a dynamic shear test that realistically simulates loading conditions created during high velocity impact, subjects the sample materials to shear and induces shear within the material at a given or chosen orientation. The shear test geometry consists of a test sample of varied geometry having a shaped surface with and without symmetry which is intended to be used in a high velocity impact test. The test sample is launched into a receptor material at various impact velocities of interest from ordnance velocity ( less than 2000 m/s) up through the hypervelocity impact range ( greater than 2000 m/s).
Conversely, reverse ballistic tests are also used wherein the receptor material is launched into a standing test sample. The shear test geometry can also employ asymmetries within the receptor to induce the desired and specific shear within the test sample. The specific orientation of the individual shear and/or shear regions are characterized using recovered samples and subsamples cut at desired orientations with respect to the shear direction. Thus, the shear morphology and microscopic characterization are uniquely related to the loading conditions of the impact and dynamic interaction as previously described, and to the orientation of the shear. The macroscopic constitutive relation is developed from the microscopic characterization of the shear.